The present invention relates to a method for preventing unwanted programming in an MRAM configuration (MRAM=magnetoresistive memory), in which memory cells in a memory cell array are situated in at least one plane at crossing points between word lines and bit lines. Programming currents are passed through the word lines, or programming lines, and bit lines associated with a selected memory cell and also produce a magnetic field in at least one memory cell which is adjacent to the selected memory cell. The magnetic field acts as a stray magnetic field in the adjacent memory cell.
FIG. 5 illustrates a so-called MTJ memory cell 1 (MTJ=Magnetic Tunnel Junction) at the crossing between a word line WL and a bit line BL running at right angles thereto. The MTJ memory cell comprises a multilayer system made up of a soft-magnetic layer (free magnetic layer) WML, a tunnel barrier layer or tunneling layer TL, and a hard-magnetic layer (permanent magnetic layer) HML. Information is then stored by changing or rotating the direction of magnetization of the soft-magnetic layer WML in relation to the direction of magnetization in the hard-magnetic layer HML. The magnetic fields required for changing the direction of magnetization in the soft-magnetic layer WML are produced by a current IWL in the word line WL and by a current IBL in the bit line BL. These magnetic fields interfere with one another at the crossing between the word line WL and the bit line BL. This is because, if the directions of magnetization in the two magnetic layers WML and HML are the same or parallel to one another, the MTJ memory cell 1 has a low resistance RC, whereas, if the directions of magnetization in the magnetic layers WML and HML are not the same or are antiparallel, the resistance RC is high (cf. the equivalent circuit diagram in FIG. 6). This resistance change, illustrated schematically in FIG. 5 by an arrow ↑ or ↓ after the symbol xe2x80x9cRCxe2x80x9d, is utilized for the purpose of storing information. In this context, to rotate or change the direction of magnetization in the soft-magnetic layer WML, it is sufficient for it to be possible to change over the direction of at least one of the currents IWL and IBL.
FIG. 6 is thus a schematic illustration of the MTJ memory cell 1 as a resistor RC between the bit line BL and the word line WL running perpendicularly thereto.
It becomes immediately obvious from FIGS. 5 and 6 that an extremely high storage density can be achieved in an MRAM configuration if a plurality of metallization systems with respective MTJ memory cells interposed are stacked above one another.
In this context, this stack can have three different array variants, shown schematically in FIGS. 7 to 9. In the variant shown in FIG. 7, the individual MTJ memory cellsxe2x80x94illustrated by resistorsxe2x80x94are arranged in a matrix directly between the word lines WL and the bit lines BL. In one such MRAM configuration, intense parasitic effects arise since, for a selected memory cell (cf. resistor in solid black), stray currents through memory cells connected to the selected word line or selected bit line cannot be avoided.
In the case of the array variants in FIGS. 8 and 9, a diode (FIG. 8) or a transistor (FIG. 9) is respectively connected in series with the individual MTJ memory cells. These array variants are much more complex, which is particularly true of the variant in FIG. 9, especially since, in that case, programming lines PRL, gate lines GL and source lines SL need to be provided in addition to the bit lines BL.
Irrespective of which of the array variants in FIGS. 7 to 9 is used for building a memory cell array for an MRAM configuration, when programming the memory cells, the appropriate bit line BL and word line WL (or programming line PRL in the array variant in FIG. 9) of the selected memory cell needs to have a respective current IBL or IWL (in a word line) impressed into it, so that the magnetic field resulting from these currents can program the selected MTJ memory cell at the crossing point between the two lines. This operation is shown schematically in FIG. 10, which illustrates crossing points between a word line WL1 and bit lines BL1, BL2 and BL3. In this case, if a current IWL flows through the word line BL1, and a current IBL2 flows through the bit line BL2, then, by way of example, the magnetic field HBL2 produced by the current IBL2 does not just influence the MTJ memory cell 12 at the crossing point between the bit line BL2 and the word line WL1. Rather, this magnetic field HBL2 also affects the MTJ memory cells 11 and 13 between the bit lines BL1 and BL3, respectively, and the word line WL1, as is shown schematically in FIG. 10.
It is thus entirely possible for MTJ memory cells situated next to the actual selected MTJ memory cell in an MRAM configuration also to be reprogrammed by stray fields resulting from the currents in the selected word line or bit line, which is referred to as a programming fault or program disturb. This applies particularly to MRAM configurations forming the multilayer systems mentioned in the introduction, that is to say specifically to the currently generally desired and sought-after high-density memory arrays with a plurality of levels of conductor tracks and interposed MTJ memory cells. Such programming faults or program disturbs are extremely undesirable in this case.
It is accordingly an object of the invention to provide a method of avoiding undesirable programming in MRAM configurations, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which can be used as a reliable and simple means of preventing stray magnetic fields from reprogramming memory cells which are adjacent to a selected memory cell.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for preventing unwanted programming in an MRAM configuration in which memory cells of a memory cell array are situated in one or more levels at crossing points between word lines and bit lines, and the memory is programmed by passing programming currents through a word line and a bit line associated with a selected memory cell to produce a given magnetic field in the selected memory cell. The method comprises:
conducting a compensating current in the memory cell array for producing a compensating magnetic field counteracting a stray magnetic field produced by the programming currents in at least one memory cell adjacent the selected memory cell.
In accordance with an added feature of the invention, the compensating current is conducted through a word line and/or a bit line associated with the at least one adjacent memory cell, or through a separate line, and the compensating current induces the compensating magnetic field counteracting the stray magnetic field.
In accordance with an additional feature of the invention, compensating currents are impressed in a respective bit line disposed next but one to a selected bit line.
In accordance with another feature of the invention, the compensating current is set to a lower value than the programming current.
In other words, the invention achieves the above objects for a method of the type mentioned in the introduction by virtue of the word line, i.e., programming line, or bit line or a separate line of the at least one adjacent memory cell carrying a current which yields a compensating magnetic field counteracting the stray magnetic field.
Hence, the inventive method uses compensating magnetic fields to prevent stray magnetic fields from influencing memory cells, which are adjacent to a programming memory cell in MRAM configurations. These compensating magnetic fields are, for their part, produced by compensating currents flowing directly in the appropriate bit lines or word lines, or programming lines, of the adjacent memory cells or else in separate lines running next to the memory cells at risk. The inventive method thus allows programming faults or program disturbs to be reliably prevented in memory cells, which are at risk.
In accordance with a further feature of the invention, the memory cell array is disposed in a multiplayer system and the conducting step comprises impressing compensating currents in word lines or bit lines in a plurality of planes.
In accordance with a concomitant feature of the invention, the level of the compensating current is advantageously controlled with self-adjusting circuits.
The novel method is particularly advantageous when it is applied to multilayer systems since, in these systems, the occurrence of stray magnetic fields is particularly problematical on account of the fact that the individual layers are very close to one another.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for preventing unwanted programming in an MRAM configuration, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.